This invention relates generally to the manufacture of circuit devices on a semiconductor wafer and more specifically to a method and apparatus for dispensing a specified amount of an acid component of an etchant bath.
In one familiar step in manufacturing a semiconductor wafer, a silicon dioxide layer on the wafer is etch with an etchant, commonly hydrofluoric acid. The etch rate varies with the concentration of the acid, and the acid is diluted with deionized water to a concentration that is appropriate for a particular etching operation.
Therefore, it is a common goal in this art to produce a solution of hydrofluoric acid and deionized water having a predefined concentration.
In a specific manufacturing technique that this invention is intended for, the wafers are placed in a bath formed by a solution of hydrofluoric acid and deionized water. This bath is held in a tank. This familiar environment will be used for illustrating the invention. The acid and the deionized water are mixed in the bath. In one practice, a specified amount of deionized water is first added to the bath and then a specified volume of acid is added.
A bath is used for a number of wafers and is then replaced. Thus, the preparation of this acid solution is a continuing task in wafer manufacture.
In the preferred manufacturing environment, the solution has 50 parts of deionized water to one part hydrofluoric acid. As a more specific example, the volume of the bath (both acid and deionized water) is 26.5 liters and the volume of hydrofluoric acid is 0.540 liters. The intended accuracy of the acid volume is +/xe2x88x925 milliliters (0.005 liter).
With this high ratio of water to acid, the desired amount of water can be measured easily with any convenient means of reasonable accuracy, but the acid volume must be established very precisely. Thus, one recognized goal in this art is to supply a precise volume of acid to the bath.
In concept, the acid could be measured by hand, using familiar laboratory measuring apparatus. However, a more automated technique is needed for a manufacturing environment.
The prior art has suggested using metering valves to supply precise amounts of deionized water and hydrofluoric acid to the tank that holds the bath. However, these valves are inaccurate if the pressure is not kept constant, and they require frequent replacement at a significant expense.
According to this invention, a tank is adapted to hold the acid and is provided with a adjustable overflow pipe. The overflow level is set to provide a volume that is slightly larger than the volume of acid that is to be supplied to the bath. The tank also has a vertically adjustable volume occupying element that reduces the volume of acid that can be held below the overflow level. The preferred volume occupying element is a rod that is threadably supported in the tank. Turning the rod raises or lowers it and thereby changes the volume at a rate that permits simple adjustments.
The tank is small and is easily located in a manufacturing line over the tank holding the acid bath.
Other objects and features of the invention will appear in the description of the preferred embodiment of the invention.